[src/trunk]: src/sys/dev/pci Issue a write barrier after updating the GTT.

[riastradh <riastradh%NetBSD.org@localhost>]

details:   https://anonhg.NetBSD.org/src/rev/fadef7c0b9df
branches:  trunk
changeset: 336331:fadef7c0b9df
user:      riastradh <riastradh%NetBSD.org@localhost>
date:      Thu Feb 26 00:42:10 2015 +0000

description:
Issue a write barrier after updating the GTT.

Linux never used to do this...until a month:

https://bugs.freedesktop.org/show_bug.cgi?id=88191

   commit 983d308cb8f602d1920a8c40196eb2ab6cc07bd2
   Author: Chris Wilson <chris%chris-wilson.co.uk@localhost>
   Date:   Mon Jan 26 10:47:10 2015 +0000

       agp/intel: Serialise after GTT updates

diffstat:

 sys/dev/pci/agp_i810.c |  11 +++++++++--
 1 files changed, 9 insertions(+), 2 deletions(-)

diffs (32 lines):

diff -r ad49739513ea -r fadef7c0b9df sys/dev/pci/agp_i810.c
--- a/sys/dev/pci/agp_i810.c    Wed Feb 25 23:14:59 2015 +0000
+++ b/sys/dev/pci/agp_i810.c    Thu Feb 26 00:42:10 2015 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: agp_i810.c,v 1.114 2014/08/24 22:56:18 riastradh Exp $ */
+/*     $NetBSD: agp_i810.c,v 1.115 2015/02/26 00:42:10 riastradh Exp $ */
 
 /*-
  * Copyright (c) 2000 Doug Rabson
@@ -30,7 +30,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: agp_i810.c,v 1.114 2014/08/24 22:56:18 riastradh Exp $");
+__KERNEL_RCSID(0, "$NetBSD: agp_i810.c,v 1.115 2015/02/26 00:42:10 riastradh Exp $");
 
 #include <sys/param.h>
 #include <sys/systm.h>
@@ -156,6 +156,13 @@
 agp_i810_post_gtt_entry(struct agp_i810_softc *isc, off_t off)
 {
 
+       /*
+        * See <https://bugs.freedesktop.org/show_bug.cgi?id=88191>.
+        * Out of paranoia, let's do the write barrier and posting
+        * read, because I don't have enough time or hardware to
+        * conduct conclusive tests.
+        */
+       membar_producer();
        (void)bus_space_read_4(isc->gtt_bst, isc->gtt_bsh,
            4*(off >> AGP_PAGE_SHIFT));
 }